I have had good luck with a 13mm flex socket and a couple of long extensions. Run the extension through the motor mounts. Works best to have a buddy work the ratchet wrench.
It is fairly easy to get one hand up there to guide the socket, but everything is done by feel. Fairly easy to tell when on the bolt.
To re-install, Lightly tape the bolt to the socket. Once the bolt is started, remove the socket and the tape. Tighten. Again, having a buddy on the wrench helps.